If your device is attached to Xcode, you can see console output (NSLogs and such) in the debug area:

If you're running the app and connecting to Xcode later, I believe you can get console logs in the Organizer.

Edit: to access the log file at runtime, you should try /var/log/system.log — but even better I recommend using a custom debug function, which would write to the system log and/or a text view in your app. (Check out NSLogv, which will be useful when writing a wrapper function.) This also has the advantage of letting you disable all debug logs from one place (just change your debug function).